Fluorescent lamps of straight tube shapes, ring shapes, and single-ended shapes are known as general fluorescent lamps, and particularly, in view of recent demands for energy conservation and conservation of resources, a small-diameter ring-shaped fluorescent lamp for high-frequency lighting has been developed and produced as a product. This small-diameter ring-shaped fluorescent lamp is identified by the name “FHC” in products (see Patent Document 1). This small-diameter ring-shaped fluorescent lamp has approximately the same external diameter for the ring size as that of conventional ring-shaped fluorescent lamps, but the external diameter of the tube is made to be smaller, while ensuring brightness equal to or greater than that of the conventional ring-shaped fluorescent lamps, thereby satisfying the needs for energy conservation and conservation of resources and, particularly, allowing a comfortable visual environment to be realized in the home space.
On the other hand, a square-shaped fluorescent lamp has been conventionally known (see Patent Documents 2 and 3). The fluorescent lamp described in the Patent Document 2 is a 30 W type square fluorescent lamp using a quadrate bulb having an external diameter for the tube of 25 to 32 mm, a curvature radius at the inner side of the bent portions of 20 to 40 mm, and an external diameter of 190 to 220 mm between opposing straight portions. The fluorescent lamp described in the Patent Document 3 is square fluorescent lamp using a quadrate bulb having an external diameter for the tube of 12.75 to 13.25 mm, an external diameter of 135 mm between opposing straight portions, and a discharge path length of 450 to 470 mm (tube length 500 to 520 mm).
[Patent Document 1] Japanese Patent No. 3055769
[Patent Document 2] Japanese Patent Laid-open Publication No. SHO 58-152365
[Patent Document 3] Japanese Patent Laid-open Publication No. HEI 3-59548
The small-diameter ring-shaped fluorescent lamp according to the Patent Document 1 is manufactured by forming a protective layer and phosphor layer in a straight tube bulb, the sealing electrodes to both ends, heating so that the entire straight tube bulb becomes soft, and bending the straight tube bulb into a ring shape, so that the initial light flux tends to be lower due to thermal deterioration of the phosphor layer. Further, alkali component in the bulb precipitates due to the heating process and reacts with the phosphor layer, thus providing problems of being easily deteriorated in time elapsing and easily deteriorating the lumen maintenance factor.
Moreover, small-diameter ring-shaped fluorescent lamps are formed into the ring shape by a straight tube bulb while being stretched in the longitudinal direction, which easily results in the cracking of the protective layer and phosphor layer formed in the straight tube at the time of shaping, thus providing a problem that the protective layer and phosphor layer cannot be formed thickly. Generally, the thicker the phosphor layer is formed, the more the initial light flux improves, and a thicker protective layer allows the luminous flux maintenance factor to be improved. However, with small-diameter ring-shaped fluorescent lamps, the protective layer and phosphor layer cannot be formed thickly by the reasons mentioned above, so that there has been a limit to the extent of improvement in initial light flux and improvement in luminous flux maintenance factor.
The square fluorescent lamp according to the Patent Document 2 simply shapes a common large-diameter 30 W fluorescent lamp into a square, and no consideration has been given to the bulb shaping process or improvement in lamp properties.
Since the square fluorescent lamp according to the Patent Document 3 has a short tube length of 500 to 520 mm, the light output is low, and high-output lighting as like as in the conventional small-diameter ring-shaped fluorescent lamps cannot be expected. Particularly, the external diameter between opposing straight portions of 135 mm is so short that a pair of electrodes must be disposed in a manner bent toward the inner side of the bulb, leading to inconveniences such that manufacturing is complicated, and that concentric combination arrangements of the same type of bulbs with the same shape but different dimensions cannot be used.
It is an object of the present invention to provide a fluorescent lamp which is small in size, highly efficient, and has improved light output properties, and a lighting apparatus using this fluorescent lamp.